Serially connected micro-inverter system having concertina output voltage control

Abstract

The present invention is directed towards a serially connected micro-inverter (SCMI) system comprising a plurality of power sources for producing DC power, a plurality of micro-inverters, where each micro-inverter is coupled to at least one power source of the plurality of power sources, for converting the DC power into AC power, an AC bus for coupling the plurality of micro-inverters in series to form a string and for coupling the AC power an AC line; and a controller, coupled to the string, for measuring an output signal of one or more strings of series coupled micro-inverters, comparing the measured output signal to a desired signal for the string; and adjusting a phase angle of an output from each micro-inverter in the one or more strings until a difference between the measured output signal and the desired signal is less than a predetermined threshold value.

The invention claimed is: 
     
       1. A serially connected micro-inverter (SCMI) system comprising:
 a plurality of power sources for producing DC power; 
 a plurality of micro-inverters, where each micro-inverter is coupled to at least one power source of the plurality of power sources, for converting the DC power into AC power; 
 an AC bus for coupling the plurality of micro-inverters in series to form a string and for coupling the AC power to an AC line, wherein each of the micro-inverters comprises (i) an AC bus coupler for coupling the micro-inverter to the AC bus, wherein the AC bus coupler decouples the micro-inverter from the AC bus when a fault is detected with the micro-inverter, and (ii) a DC to AC inverter for converting DC input power from a coupled power source to AC output power; and 
 a controller, coupled to the string, for:
 measuring an output voltage of the string; 
 comparing the measured output voltage to a desired voltage for the string; and 
 adjusting a phase angle of an output from each micro-inverter in the string until a difference between the measured output voltage and the desired voltage is less than a predetermined threshold value. 
 
 
     
     
       2. The SCMI system of  claim 1 , wherein each of the micro-inverters further comprises:
 a current-voltage monitoring circuit for monitoring current and voltage from a corresponding power source; and 
 a maximum power point tracking controller for maintaining maximum power output from the micro-inverter. 
 
     
     
       3. The SCMI system of  claim 2 , wherein each of the micro-inverters further comprises:
 a local controller that synchronizes phase of an AC output from the micro-inverter and voltage phase of the AC line; and 
 
       a control bus coupler for receiving information regarding the micro-inverter from the local controller. 
     
     
       4. The SCMI system of  claim 3 , wherein the local controller further comprises:
 an inverter controller for controlling parameters of the DC to AC inverter. 
 
     
     
       5. The SCMI system of  claim 1 , wherein each micro-inverter of the plurality of micro-inverters is a voltage source inverter (VSI). 
     
     
       6. The SCMI system of  claim 5 , wherein the VSI comprises:
 an input capacitor; 
 an H-bridge, coupled to the input capacitor; and 
 a plurality of output inductors to generate an AC output voltage. 
 
     
     
       7. The SCMI system of  claim 1 , wherein each micro-inverter of the plurality of micro-inverters is a current source inverter (CSI). 
     
     
       8. The SCMI system of  claim 7 , wherein the CSI comprises:
 a plurality of inductors; 
 an H-bridge, coupled to the plurality of inductors; and 
 an output capacitor, coupled to the H-Bridge, to generate a pulsed AC current signal. 
 
     
     
       9. The SCMI system of  claim 1  wherein the plurality of power sources comprises a plurality of photovoltaic (PV) modules that generate power when exposed to solar irradiance. 
     
     
       10. The SCMI system of  claim 1 , wherein the controller is coupled to the Internet for remote monitoring and control. 
     
     
       11. The SCMI system of  claim 1 , wherein a plurality of strings are coupled in parallel to generate an output power represented by the equation: P o (t)=Σ m (i m (t)·Σ m v n (t)) where P o (t) is the output power generated by the SCMI system, I m (t) is current in a given string from the plurality of strings and V n (t) is voltage produced by an individual micro-inverter. 
     
     
       12. The SCMI system of  claim 11 , wherein power generated by a string is represented 
       
         
           
             
               
                 P 
                 ⁡ 
                 
                   ( 
                   t 
                   ) 
                 
               
               = 
               
                 
                   
                     i 
                     m 
                   
                   ⁡ 
                   
                     ( 
                     t 
                     ) 
                   
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 • 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   
                     ∑ 
                     n 
                   
                   ⁢ 
                   
                     
                       v 
                       n 
                     
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                 
               
             
           
         
       
       by the equation: where I m (t) is the current in the string and V n (t) is the voltage produced by an individual micro-inverter. 
     
     
       13. A method for controlling output voltage of a serially connected micro-inverter system comprising:
 measuring an output voltage of a string of series coupled micro-inverters coupled to an AC bus, wherein each of the micro-inverters comprises (i) an AC bus coupler for coupling the micro-inverter to the AC bus, wherein the AC bus coupler decouples the micro-inverter from the AC bus when a fault is detected with the micro-inverter, and (ii) a DC to AC inverter for converting DC input power from a coupled power source to AC output power that is coupled to the AC bus; 
 comparing the measured output voltage to a desired voltage for the string; and 
 adjusting a phase angle of an output from each micro-inverter in the string until a difference between the measured output voltage and the desired voltage is less than a predetermined threshold value. 
 
     
     
       14. The method of  claim 13  further comprising:
 waiting, according to a sampling rate, a period of time until measuring the output voltage again to determine the difference between the measured output voltage and the desired voltage. 
 
     
     
       15. The method of  claim 14 , wherein the period of time is once per grid cycle. 
     
     
       16. The method of  claim 13 , wherein the desired voltage is one of 120 volts, 220 volts, or 240 volts. 
     
     
       17. The method of  claim 13 , wherein adjusting the phase angle of the output from the string drives a sum of real signal components over the string towards the desired voltage.